HID lamps require a ballast in the electric circuit with the lamp in order to operate. The ballast supplies the requisite open circuit voltage to start and maintain an arc in the discharge tube as well as limiting the current therethrough. One style of ballast uses a high voltage pulse to initiate breakdown in the arc discharge tube. Arc tube breakdown is the first phase of lamp starting and is, therefore, essential for lamp operation. The typical high voltage pulse for a ballast of this type has an amplitude between 3.0 and 4.0 kilovolts with a width of 1.0 .mu.s at 2.7 kilovolts. The maximum voltage can be increased; however, such an action requires a more expensive base on the lamp and a more expensive socket in the fixture.
There are two commercial ballast methods of applying the typical voltage to the lamp. The first method applies the pulse voltage to the center contact of the lamp base. The second method divides the pulse between the center contact and the shell of the base. The second method, referred to as the split lead design, has the unusual characteristic of floating the lamp lead wires such that both lamp lead wires carry pulse voltage with respect to ground. When the pulse voltage is applied to the lamp, 1.7 kv is applied to the center contact and an opposite potential of approximately equal magnitude is applied to the shell of the lamp.
With the typical high voltage pulse, the HID lamp requires a starting aid to initiate instantaneous breakdown. There are several known aids to reduce the pulse requirements and to make less expensive systems. One such aid adds radioactive krypton 85 to the argon gas contained in the arc tube. Another method is to lower the arc tube buffer gas pressure. Still another employed technique adds a glow bottle that produces ultraviolet light adjacent the arc tube (see U.S. Pat. No. 4,818,915). Yet another technique adds a bubble containing gas to the arc tube seal. This technique also produces ultraviolet light in the area of the arc tube. (See, U.S. Pat. No. 5,323,091 and Ser. No. 08/372,069).
While each of these techniques is workable, they all have some disadvantages. The use of krypton 85 has the attendant difficulties of handling radioactive materials such as gas reclaiming systems and the cost of state and federal licenses.
The use of glow bottles is not advantageous because the glow bottle would have to be inside the aluminosilicate jacket and there simply is not enough room. Positioning outside of the jacket is not workable since the aluminosilicate jacket does not pass UV radiation.
The bubble-in-the-press approach requires extra molybdenum foils to penetrate the bubble. This is difficult to accomplish in the smaller size arc tubes and adds material cost as well as assembly cost. Further, this technique is not workable with a split lead ballast since the voltage applied to the bubble electrode would only be 1.7 kv with respect to the isolated frame and the bubble gas would not breakdown.